It now needs to be done fully from System Preferences > Internet accounts
Ignore the Google option as this no longer has the Messages option (It uses a different server which is why it cannot be linked to the Mail login)
Add an Other Account then choose Messages.
You get to choose Jabber and enter your Google ID. (It says you can add AIM but this does not work form the Messages end and AIM are ending the service in December and have done already for client apps like iChat and Messages).
Then go to Messages and the Messages Menu > Status and change it to Available as this make s the window potentially visible
Use the Window Menu which will now include "Buddies"
Changing the Description in Internet Accounts will change it in Messages
9:25 pm Thursday; November 16, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)